1. Field of the Invention
The invention is directed to optical transport networks and in particular to dispersion management for long-haul, high-speed optical networks.
2. Background Art
In communication systems which utilize optical fiber as a transmission medium, chromatic dispersion and fiber nonlinearities present significant obstacles to achieving higher system data rates and longer transmission distances (reach). “Chromatic dispersion” is the linear, power-independent dependence of light velocity on wavelength. The dispersion is measured in pulse spread in pico seconds, for a certain wavelength in nanometers, over a distance in kilometers (ps/nm/km).
Typically, the diameter of the optical cable is about 125 microns but the core itself comes in to two different sizes, depending on the application the fiber is intended for. The fiber with a core diameter of 50 microns is known as multimode fiber, and the fiber with a core diameter of 8.6–9.5 microns is known as single mode fiber.
Multimode fibers support the fundamental mode or higher modes, the number of modes depending on the wavelength and core size. Multimode fiber accommodates rates of up to 100 Mbps for a distance of up to 40 km; shorter lengths support higher speeds.
Single mode fiber support the fundamental mode (one mode or one channel), which travels along its longitudinal axis. Single mode fiber is suitable for transmitting modulated signals at 40 Gbps or more and up to 200 km without amplification. On the other hand, the single mode fiber is more difficult to splice and to connect light into it, due to the smaller diameter of the core.
Chromatic dispersion is particularly significant in the standard single-mode fiber (SMF), making up much of the world's existing optical fiber infrastructure. Standard SMF typically exhibits minimum dispersion (about zero) at a wavelength of about 1300 nm (first telecommunication window). Below this point dispersion is negative, and above is positive. This means that SMF presents positive dispersion for wavelengths in the 1550 nm band (the second telecommunication window), which is currently preferred for wavelength division multiplexed (WDM) systems.
Fiber manufacturers are looking to produce optical cables adapted to specific applications, as long haul versus short haul systems, DWDM versus single channel transmission, unidirectional versus bidirectional, etc. For example, dispersion-shifted fiber (DSF) is obtained by displacing the minimum dispersion from the first window to the second, this type of fiber being compatible with optical amplifiers that perform best around 1550 nm.
There are other types of fibers, such as dispersion compensated fiber (DCF), with a refractive index profile that has an opposite effect on a specific range of wavelengths than conventional fibers. The DCF's are currently used in dispersion compensating modules (DCM), which are combined with a fiber amplifier needed to overcome the attenuation introduced into the system by the DCF.
Dispersion flattened fiber (DFF) has near zero dispersion in the range from 1300 nm to 1550 nm. Depending on the dispersion slope, there is positive DFF, negative DFF and dispersion flattened compensated fiber (DFCF).
Compensation of chromatic dispersion can also be provided utilizing any of a number of nonlinear conversion media including, for example, semiconductor lasers, semiconductor laser amplifiers and nonlinear crystals.
Development of high rate DWDM (dense wavelength division multiplexed) systems using NZDSF (non-zero dispersion shifted fiber) is a competitive advantage for the new common carriers. However, the production rates of NZDSF are slower than those of SMF, adversely affecting the industry ability to meet demand.
Chromatic dispersion plays a significant role in WDM systems, and particularly in the emerging DWDM systems, since silica, a key ingredient of optical fiber cable, has a refractive index that varies with optical frequency. A key advance in the implementation of multi-channel WDM systems is “dispersion management”, which becomes more complex with the increase of the number of channels transmitted on the same fiber. The basic principle of dispersion management is to keep local dispersion non-zero (to reduce non-linear effects such as four-wave-mixing), while making overall system dispersion substantially zero (to minimize pulse spreading, with the resulting intersymbol interference).
Dispersion slope compensated fiber (DSCF or SCF) are manufactured to compensate the dispersion for a group of wavelengths, therefore being suitable for WDM transmission.
As indicated above, SMF presents in general a positive dispersion and dispersion slope in the 1520–1565 nm region, which means that the dispersion of each channel grows with the channel wavelength. SCF is manufactured with a positive slope for the same region, to compensate for the dispersion versus wavelength slope of the SMF fiber.
One dispersion management scheme uses a conventional fiber with positive dispersion (about 2 ps/nm-km in the 1550 nm window); in this case the cumulative dispersion after a few hundred kilometers reaches several hundred ps/nm-km. This dispersion can be compensated with an approximately equal length of fiber having a corresponding negative dispersion (about −2 ps/nm-km).
In another scheme, dispersion accumulated along a conventional fiber with zero dispersion at 1310 nm and a dispersion of +16 ps/nm-km at 1550 nm can be compensated with a DCF with high negative dispersion. To compensate for this value, an appropriate length (e.g., about 10 km) of the DCF is inserted into the transmission path every 50–60 km. The dispersion of the DCF is more than about −90 ps/nm-km.
Prior art dispersion management schemes, while being effective for single channel fiber communication systems, have at least one shortcoming with regard to multichannel systems. Specifically, complete cancellation of dispersion in all channels at the end of the system is not easily accomplished, primarily because the dispersion slope in the compensating fibers typically cannot meet the two requirements of being high in magnitude and negative in sign. Thus, fibers with high negative dispersion and high negative slope are difficult to manufacture and therefore expensive. Small variations in fiber designs having these properties typically lead to large changes of other properties of the fiber, and hence such designs are typically not reliably manufacturable. Also, there is a large installed base of SMF fiber, and even if SCF were easier to manufacture, replacing of the existing outside cable plant would be very costly.
As discussed above, dispersion compensation results in adding attenuation to the signal, which limits the possible spacing of the terminals. To achieve long haul transmission at high line rates, regenerators (repeaters) and/or Erbium Doped Fiber Amplifiers (EDFA) are deployed along the optical transmission link in multiple locations, for boosting the signal on the fiber. One of the major advantages of the EDFAs is that they amplify whatever bit rate comes down the fiber. Typically, the distance between the amplifier sites is in the range between 80 to 160 Km. This distance is determined by the optical power launched into the fiber by the upstream amplifier, the loss and dispersion of the optical fiber interconnecting this amplifier with the closest downstream amplifier, and the sensitivity of the downstream amplifier.
On the other hand, EDFA's are expensive units and so the number and types of units required to implement a given data connection is an important design parameter for an optical network. Furthermore, the EDFA gain is not perfectly flat for all wavelengths, the precise wavelengths to use being a function of the gain variations of the different available pumps. Gain tilt is a significant issue when designing DWDM systems. Gain tilt measures the change in the profile of the gain for each transmission channel at the actual value of the gain of the amplifier module, with respect to the gain profile at the nominal value of the gain, i.e. at the value for which the amplifier is designed. In other words, the gain tilt function varies with link loss.
Another type of optical amplification which is gaining ground over the EDFA in WDM systems is based on a phenomenon known as Raman gain. The Raman gain effect is an interaction between light and molecular vibrations (in this case of Silicon and Oxigen ions in the glass) which is used to make an optically-pumped optical amplifier.
To amplify the signal using Raman gain, a pump signal, whose wavelength is less than the wavelength of all components of the DWDM signal, is pumped into the optical fiber in a direction against the traffic. The pump amplifies the DWDM signal, and thus offsets the insertion loss.
Use of a pump source rather than an EDFA results in considerable savings. Raman pumping is very efficient, so that required pump powers are readily obtained from semiconductor laser diodes of commercial design. In typical fibers, a factor of 10 amplification would require 1 W of pump power and a fiber 1 km long. Raman pumping retains the advantages that led to adoption of the EDFA for use in 1550 nm systems. Also, the Raman passband is of sufficient breadth for simultaneous amplification of member channels. Furthermore, Raman amplification is relatively uniform across the band.
There is a need to provide a dispersion managed solution for long-haul, high-rate optical transmission systems.